6mpa on the train for an airport that will serve 40mpa. Seems a bit high based on experience elsewhere, however if you are trying to get the bus frequency from 10min to 5min if thats not evidence you need a rail line I don't know what is.
6mpa, x $15/head = $90m pa in revenue, thats quite a reasonable rate of a return for a govt still subsidised railway that will I assume cost between $1B and $1.5B (guessing) to build. Other revenue to other stations as well as shops etc will only bolster the returns.
That's a roughly equivalent market share to Sydney and about the same fare for a much greater distance. I actually think this is pessimistic.
Sydney Int entries: 2080/weekday
Sydney Dom entries: 5160/weekday
= 4054400 trips per year (280 days/yr, 2 trips per entry).
SYD airport = 33m pax/yr.
Share = 12.3%